Statoil’s pioneering subsea developments continue, with the completion of the world’s first subsea wet gas compressor at the Gullfaks field
October has signalled yet another major month for Norwegian NOC Statoil. Even in a year already full of massive contract announcements and development milestones, the completion and installation of a world-first piece of subsea engineering should still prove a highlight.
Following the installation of a similar pioneering subsea compression system for dry gas at its Åsgard field in September, Statoil, Petoro and OMV have completed the commissioning of a wet gas compressor at the Gullfaks South satellite field. The system was first installed for testing to begin in March this year, and the October 12 announcement by Statoil marks the completion of efforts dating back to 2012.
The 950-tonne system will compress wet gas and liquids together on the seabed at a depth of 135m, without the need for separation. Designed by Framo Engineering and delivered by OneSubsea in collaboration with Statoil and Shell, the set-up features “a contra-rotating machine specifically designed for pressure boosting of unprocessed wellstream.”
With both phases treated in the same system, and as close to the well as possible, overall production is greater, faster and more efficient, from a smaller and lighter subsea template. This compact design allows the compressor to be installed by an light intervention vessel (LIV) rather than a larger constructor.
Following compression, gas is then sent through a 15-km tieback to the main Gullfaks C platform where it can be processed. The platform also supplies power and controls to the template’s two 5-MW compressors. These are capable of compressing the gas and liquid to 32 or 60 bar – depending on whether they are run in parallel or connected in series – and can handle up to 10 million cubic metres per day. OneSubsea also reports that the lack of a surge risk means that no anti-surge system is needed, again lowering the overall weight and footprint.
The factory floor
The system, and Statoil’s wider push towards what it calls the “subsea factory”, is an important step for both engineering and overall recovery hydrocarbon recovery. According to Statoil senior vice president for west cluster operations, Kjetil Hove, “This is one of several important projects on Gullfaks for improved recovery and field life extension. The recovery rate from the Gullfaks South Brent reservoir may be increased from 62% to 74% by applying this solution in combination with other measures.”
In figures, it estimates this at around 22 million barrels of oil equivalent (3 bcm gas). As well as extending the life of fields like Gullfaks and Åsgard – the latter’s compressor is touted to boost recovery from the Mikkel and Midgard reservoirs by 306 million boe – technologies like this will be vital in meeting Statoil’s goal of raising average Norwegian recovery rates to 60%.
“We see great opportunities for wet gas compression on the Norwegian Continental Shelf [NCS],” Hove added. “It is an efficient system and a concept that can be used for improved recovery on small and medium-sized fields. We are searching for more candidates that are suitable.”
These additional candidates could also be tied into the Gullfaks facility, which has been designed to incorporate potential additional wells in future. This feature is also indicative of the wider push for standardisation, a move hinted at by Statoil’s executive vice president for technology, projects & drilling (TPD), Maragareth Øvrum, who commented: “The concept may be standardised by applying well-known technology components.”
For now, next year might be a little quieter, as development efforts at Johan Sverdrup continue. A Statoil TPD spokesman confirmed to InnovOil: “We have no specific development plans which include the realisation of subsea compression in 2016.”
Yet its potential is significant. The spokesman concluded: “Based on the experience achieved through [Gullfaks and Åsgard], subsea compression will be considered for use in the future, both on the NCS and in other basins.”